There are a variety of different technologies which employ one kind or another of a type of liquid-solid mixture termed a "slush". Generally, a slush is a liquid-solid mixture in which the solid is the frozen form of at least a portion of the liquid part of the mixture. The liquid is a continuous phase, while the solid is a discontinuous phase composed of a multitude of discrete particles, and the mixture is flowable and/or pumpable. Slushes and the means of producing them are well known in the art. Illustrative of slushes, the particular techniques for making them and their uses or employments are U.S. Pat. No. 4,300,355, which describes a water-ice slush and a use therefor, and the paper entitled "Slush Hydrogen Production for the National Aero-Space Plane Programs" by Robert B. Moore, Glenn E. Kinard and David Nahmias presented at the World Hydrogen Energy Conference #8, 22-27 July 1990, in Hawaii, which describes a liquid-solid hydrogen slush and a use therefor. The techniques for making slushes all require the use of "low" temperatures--some at about the freezing point of water and others requiring temperatures in the cryogenic range. As will be readily understood, the use of any such slush for its intended purpose immediately after its preparation is rather straightforward. A problem arises, however, when the slush can not or is not used immediately and the nature of the problem is exacerbated by the differential between the temperature of the slush and ambient temperature. In the case of the ice-water slush described in U.S. Pat. No. 4,300,355, it is quite common for the differential between the slush and ambient temperatures to range from 50.degree. to 70.degree. F. during the summer months in the United States. Thus, for example, if the slush of the U.S. Patent is not employed immediately, there will be a gradual melting of the ice and loss of the greater cooling capacity of the slush, along with concomitant reduction in the advantages of this particular slush employment.
When the slush hydrogen of the Moore et al. paper is considered, it will be seen that the temperature differential between slush and ambient is generally in the range of about 500.degree. F. In this case, the nature of the application precludes a high degree of insulation and the rate of melting of the solid in the slush will be extremely rapid when the slush is not used immediately.
For hypersonic and space plane applications, a probable fueling procedure requires that the vehicle be fueled with slush hydrogen at a fixed fueling station, after which the vehicle will be towed to the end of a runway--a trip lasting 30 minutes or more. Because of weight and space limitations in a practical flight vehicle, thermal insulation of the fuel tank is inadequate to prevent substantial melting of the solid component of the slush hydrogen before the vehicle becomes fully operative.
It has been suggested that a portable "reslushifier" be towed along with such a vehicle in order to keep the vehicle fuel tanks at design conditions. This is not seen as being feasible, since the slush replenishment rate is on the order of 100 times greater than the production rate for the fixed ground support system slush generators.
Simply adding extra slush hydrogen to the fuel tank and permitting the solids in the slush to melt is also not a satisfactory solution, since this would require designing oversized fuel tanks with excessive initial ullage to allow space for expansion of the fuel as the solids melt, and would diminish the ability to utilize the fuel as a coolant to the degree desired during take off and flight.